What is the difference between doing low rep/heavy weight VS high rep/lighter weight?

I'm not sure what the difference is between these two methods of lifting. From my understanding, low rep/heavy weight will help to make gains in size and strength. High rep/lighter weight is for getting really defined muscle. Thanks for any feedback!

You basically got it right. Low weight/high reps is better for burning fat while high weight/low rep helps with strength and size. It's best to figure out what your main goal is and go from there. You can also mix and match. I did a 3 day a week workout for a while where Day 1 (Mon) I lifted heavy weight/low rep, Day 2 (Wed) was medium weight/medium reps, and Day 3 (Fri) was lower weight/higher reps. The workout consisted of Bench PressSquatBarbell RowStiff-legged DeadliftShoulder PressCalf-raisesBiceps CurlTricep Rope Extension

Whatever you choose to do make sure you enjoy it. And when you start to get bored, mix it up!